Process for preparing melamine

ABSTRACT

The invention relates to a process for preparing melamine, comprising a cooling step from which a mixture H 2 O containing NH 3 , CO 2  and H 2 O is released, wherein: 
         the mixture and a flow of liquid NH 3  are fed to an absorption section;    the mixture and the flow of liquid NH 3  in the absorption section are contacted with each other, with gaseous NH 3  and an ammonium carbamate solution being formed;    the gaseous NH 3  and the ammonium carbamate solution are separately discharged from the absorption section, 
 
wherein a part of the discharged ammonium carbamate solution is returned to the cooling step and a part of the removed gaseous NH 3  is liquefied and returned to the absorption section.

The invention relates to a process for preparing melamine, comprising afirst cooling step from which a mixture containing NH₃, CO₂ and H₂O isreleased, wherein:

-   -   the mixture and a flow of liquid NH₃ are fed to an absorption        section;    -   the mixture and the flow of liquid NH₃ in the absorption section        are contacted with each other, with gaseous NH₃ and an ammonium        carbamate solution being formed;    -   the gaseous NH₃ and the ammonium carbamate solution are        separately discharged from the absorption section.

A process of the same kind is known from “Melamine and Guanamines”,section 4.1.3 of Ullmann's Encyclopedia of Industrial Chemistry, SixthEdition, 2001 Electronic Release. Furthermore, in the known process, themixture, prior to being supplied to the absorption section, is subjectedto a partial condensation step.

The mixture in the known process contains, as mentioned above, NH₃, CO₂and H₂O. The CO₂ and a part of the NH₃ in the mixture mainly originatefrom the reaction of urea to form melamine, which proceeds as follows:6 CO(NH₂)₂-->C₃N₆H₆+6NH₃+3CO₂

Usually NH₃ is used as fluisidation gas and atomizing gas during themelamine synthesis, which may also cause NH₃ to enter the mixture.Fluidization gas serves to keep the synthesis catalyst in a fluid state.Atomizing gas serves to atomize the urea in the synthesis reactor. TheH₂O in the mixture mainly originates from the first cooling step, wherean aqueous flow is used as a cooling medium for the reaction product;here the H₂O partly evaporates.

The aforementioned known absorption section, which is designed as acolumn, is intended amongst other things to separate a flow of gaseousNH₃ from the mixture and discharge it from the absorption section. Thedischarged flow of gaseous NH₃ is removed and generally returned to amelamine synthesis reactor to serve there as a fluidization gas and/oratomizing gas. A part of the gaseous NH₃ is passed through a scrubber,whose object it is to to separate and discharge so-called inertcompounds such as H₂ and N₂.

The known absorption section is also intended to form, from NH₃ and fromvirtually all CO₂ and H₂O from the mixture, an ammonium carbamatesolution and discharge that solution. The discharged ammonium carbamatesolution is removed, after which generally in downstream steps the CO₂and the NH₃ are liberated from the discharged ammonium carbamatesolution, either as such or in the form of an ammonium carbamatesolution with a considerably reduced water content, in which process apractically pure flow of water is released which in general is a wastestream and is discharged. As is known, the urea synthesis efficiencydecreases as the quantity of water in the raw materials increases. Theliberated CO₂ and a part of the liberated NH₃, as such or in the form ofan ammonium carbamate solution with a considerably reduced watercontent, can serve as a raw material for the production of urea or forthe production of ammonium nitrate or ammonium sulfate. The remainingpart of the liberated NH₃ is liquefied in the known process andrecirculated to the absorption section as part of the flow of liquidNH₃.

A disadvantage of the known process is that the recovery of the NH₃ tobe recirculated to the absorption section from the ammonium carbamatesolution requires a great deal of energy, for example in the form ofsteam.

The object of the invention is to reduce the energy consumption in thedownstream steps that are applied to the discharged ammonium carbamatesolution.

The said object is achieved in that a part of the discharged ammoniumcarbamate solution is returned to the first cooling step and a part ofthe discharged gaseous NH₃ is liquefied in a second cooling step andreturned to the absorption section.

The process according to the invention ensures that less NH₃ is removedvia the ammonium carbamate solution per unit time than in the knownprocess. As a result, the quantity of NH₃ which is liberated in thedownstream steps for recirculation to the absorption section candecrease, which yields an energy saving, for example in steamconsumption. A further advantage of the process according to theinvention is that less H₂O is removed per unit time via the ammoniumcarbamate solution than in the known process. This allows a decrease inthe liquid load of the equipment items in which the downstream steps arecarried out for liberating the CO₂ and the NH₃ from the removed ammoniumcarbamate solution, for example for the preparation of urea. As a resultsaid downstream steps can be carried out in smaller equipment items andwith less energy consumption than in the known process, which ischeaper.

The process according to the invention comprises a first cooling stepfor cooling an essentially gaseous flow originating from a melaminesynthesis reactor and containing essentially melamine, NH₃ and CO₂. Theflow from the melamine synthesis reactor is contacted with a coolant.The coolant comprises a flow consisting essentially of an ammoniumcarbamate solution to be discussed later. The coolant comprises inaddition generally also a flow essentially consisting of water; for thispurpose use is usually made of an aqueous flow originating from arecovery section. The coolant flows can be supplied separately orjointly to the first cooling step. As the flow from the melaminesynthesis reactor is contacted with the coolant there evolve a solution(containing dissolved melamine in an aqueous phase) or a slurry(containing melamine crystals in an aqueous phase) and a generallygaseous mixture containing NH₃, CO₂ and H₂O. The solution or the slurryis discharged from the first cooling step and fed to a recovery sectionin which melamine crystals are formed (if necessary) and separated. Thesaid mixture is usually gaseous and usually contains more than 40 wt %NH₃, less than 50 wt % CO₂ and less than 40 wt % H₂O at a pressureusually between 0.1 MPa and 4 MPa, and is fed to an absorption section.In addition a flow of liquid NH₃ is also fed to the absorption section.The percentages stated here and hereafter are percentages by weight,unless otherwise indicated.

In the absorption section according to the invention the mixture iscontacted with the flow of liquid NH₃. This gives rise to a flow ofgaseous NH₃ and virtually all CO₂ and H₂O go into the liquid phase. Tofacilitate the transition of CO₂ into the liquid phase, an additionalflow consisting essentially of water may be fed to the absorptionsection. The required quantities of the flow of liquid NH₃ and of theadditional flow consisting essentially of water that are fed to theabsorption section are largely determined by the desired purity of thedischarged flow of gaseous NH₃. Usually a purity of 99% or higher isdesired. The higher the desired purity of the discharged gaseous NH₃,the larger the required quantities of the flow of liquid NH₃ and of theadditional flow consisting essentially of water.

The flow of gaseous NH₃, consisting essentially of NH₃ and which inaddition may contain up to approximately 1 wt % of other compounds suchas CO₂, H₂O, N₂ and H₂, is discharged from the absorption section. Inthe process according to the invention a part of the discharged gaseousNH₃ is liquefied in the second cooling step and returned to theabsorption section; the remaining part is removed and, as previouslyindicated, usually used as fluidization gas and/or atomizing gas in themelamine synthesis reactor. It is an advantage of the process accordingto the invention that the gaseous NH₃ can be used directly asfluidization gas an/or atomising gas, without the need of an evaporatingstep as is the case when the NH₃ to be used is supplied in liquid form.Liquefication in the second cooling step may be accomplished by meansknown per se, such as for example by means of a heat exchanger. Anadvantage of the process according to the invention is that the quantityof fresh liquid NH₃ to be fed to the absorption section can decrease,because a flow of liquid NH₃ is already available. A further advantageof the process according to the invention is that inert compounds suchas for example N₂ and H₂, which cannot be easily liquefied because theircondensation temperature is much lower than that of NH₃, can readily beseparated from the liquefied NH₃ and removed as an inert gaseous ventstream. The inert vent stream will in general still contain a residualquantity of NH₃. If it is desired to remove this residual quantity ofNH₃ the inert vent stream can be passed through a scrubber, which canhowever be significantly smaller than the aforementioned scrubber in theknown process.

The liquid phase formed in and discharged from the absorption sectionconsists essentially of an ammonium carbamate solution. By this is meanta solution of ammonium carbamate in water, which in addition may containother compounds such as free dissolved NH₃, free dissolved CO₂ andammonium bicarbonate. In the process according to the invention a partof the discharged ammonium carbamate solution is returned to the firstcooling step; the remaining part is removed. The returned ammoniumcarbamate solution serves as a coolant in the first cooling step, willlargely evaporate into NH₃, CO₂ and H₂O and will thus be absorbed in themixture which is released in the first cooling step and is fed to theabsorption section; in this way there is formed in the process accordingto the invention a circulation flow between the first cooling step andthe absorption section.

The removed ammonium carbamate solution is generally the most importantmedium for removing the CO₂ evolving in the urea-to-melamine reaction.As a result, the quantity of CO₂, as such or as an ion, in the removedammonium carbamate solution is usually an important control parameterfor operating an absorption section in a process for the preparation ofmelamine and also for operating the absorption section according to theinvention. The part of the discharged ammonium carbamate solution thatis returned to the first cooling step is determined preferably inrelation to the quantity of CO₂ that is present in the removed ammoniumcarbamate solution either as such or as an ion. The quantity of CO₂ inthe removed ammonium carbamate solution is hereinafter referred to asremoved CO₂. The quantity by weight of ammonium carbamate solution whichis returned to the first cooling step divided by the quantity by weightof removed CO₂ is preferably between 0.01 and 5, more preferably between0.3 and 2.0 and most preferably between 0.7 and 1.7.

The part of the discharged gaseous NH₃ which is liquefied can also beexpressed in relation to the quantity of removed CO₂. The quantity byweight of discharged gaseous NH₃ which is liquefied divided by thequantity by weight of removed CO₂ is preferably between 0,01 and 5, morepreferably between 0.1 and 2.0 and most preferably between 0.5 and 1.5.

The absorption section may be of any known design, such as for example atrayed column so configured that the formed ammonium carbamate solutionand the formed flow of gaseous NH₃ can be discharged separately.

In order to form the ammonium carbamate solution in the absorptionsection it is generally necessary to withdraw heat from the mixture.Therefore, prior to being fed to the absorption section, it is preferredto subject the mixture to a third cooling step, wherein partialcondensation takes place whereby a liquid phase and a gas phase areformed. The liquid phase then consists essentially of an ammoniumcarbamate solution. In a further preferred embodiment the liquid phaseis separated from the mixture. It is then possible to remove this liquidphase separately from the ammonium carbamate solution coming from theabsorption section or to wholly or partially return it to the firstcooling step. An advantage of the performance of the said third coolingstep is that the operation of this step appears to be a control tool indetermining the quantity of gaseous NH₃ that is released from theabsorption section as a consequence of the fact that not all compoundsof the mixture condense proprotionately; particularly H₂O condensespreferentially, which makes it easier to separate gaseous NH₃ in theabsorption section.

In another preferred embodiment of the process according to theinvention a compression step is carried out on least the part of thedischarged gaseous NH₃ that needs to be liquefied in the second coolingstep. This presents the advantage that the condensation temperature ofthe gaseous NH₃ rises, which simplifies condensation. Preferably thegaseous NH₃ pressure is increased to at least 1.5 MPa, more preferably1.8 MPa or even higher, for example 2 MPa or more. This presents theadvantage that the condensation temperature of NH₃ at the statedpressures exceeds the temperature level at which plant cooling watercircuits usually operate. This preferred embodiment can readily becombined with the aforesaid preferred embodiments such as theapplication of partial condensation.

The ammonium carbamate solution being formed in the absorption sectionhas an amount of NH₃ and NH₃-derived compounds such as ammonium ions,and an amount of CO₂ and CO₂-derived compounds such as carbamate- orcarbonate ions. The weight ratio between these two amounts is expressedas the N/C ratio of the ammonium carbamate solution. In a furtherembodiment of the process according to the invention, the N/C ratio inthe ammonium carbamate solution is reduced to 1.4 or less by returning apart of the discharged ammonium carbamate solution to the first coolingstep and by subjecting the gaseous NH₃ and/or the mixture to a second,respectively third cooling step. Preferably, the N/C ration is reducedto 1.3 or less, more preferably to 1.25 or less, most preferably to 1.2or less. In this embodiment, it is preferred that the third cooling stepcomprises a partial condensation step on the mixture as describer above,prior to the mixture being fed to the absorption section, with a liquidphase and a gas phase being formed. Preferably, the liquid phase issubsequently separated from the mixture. Advantageously, a part of thedischarged gaseous NH₃ is liquefied in the second step and returned tothe absorption section, as described above. Here, it is preferred asdescribed above that a compression step is carried out on at least thepart of the discharged gaseous NH₃ that is to be liquefied.

The process according to the invention is elucidated on the basis of thefollowing drawings.

In the drawings FIG. 1 shows an embodiment with a cooling vessel inwhich the first cooling step is carried out, an absorption sectionconsisting of an absorber, a condenser wherein the gaseous NH₃ isliquefied in the second cooling step and wherein the gaseous NH₃ and theammonium carbamate solution are returned to the absorber and the coolingvessel, respectively.

FIG. 2 shows an embodiment wherein, in comparison with the embodiment ofFIG. 1, the mixture from the first cooling step is first passed througha condenser, where the third cooling step is executed, before being fedto the absorber.

FIG. 3 shows an embodiment wherein, in comparison with the embodiment ofFIG. 2, the liquid phase is separated from the flow coming from thecondenser placed between cooling vessel and absorber and is added to theammonium carbamate solution coming from the absorber.

FIG. 4 shows an embodiment wherein, in comparison with the embodiment ofFIG. 1, the part of the flow of gaseous NH₃ from the absorber that needsto be liquefied is compressed in a compressor, prior to the liquefactionin the second cooling step.

FIG. 5 shows an embodiment according to the state of the art.

The first digit of the numbers in the figures is the same as the numberof the figure. Where the last two digits of the numbers of differentfigures are the same, they refer to the same item.

In FIG. 1 a gaseous flow coming from a melamine synthesis reactor is fedvia line 102 to cooling vessel 104 and cooled with a coolant consistingof an aqueous flow supplied via line 106 and an ammonium carbamatesolution, to be discussed later, which is supplied via line 108. Duringcooling there are formed a melamine slurry, which is discharged via line110, and a gaseous mixture containing NH₃, CO₂ and H₂O, which isdischarged via line 112 to absorber 114. Absorber 114 is supplied with aflow of liquid NH₃ via line 116 as well as an auxiliary flow, consistingessentially of water, via line 118. It is possible to combine lines 116and 118 into one line. Contacting the mixture with the liquid NH₃ andthe auxiliary flow results in the formation of a flow of gaseous NH₃,which is discharged via line 120, and an ammonium carbamate solution,which is discharged via line 122. The ammonium carbamate solutiondischarged via line 122 is split: one part is removed via line 124, theother part is returned via line 108 to cooling vessel 104. The gaseousNH₃ discharged via line 120 is split: one part is removed via line 126and generally used as fluidization gas and/or atomizing gas for themelamine synthesis reactor, the other part is supplied via line 128 tocondenser 130. In condenser 130 the NH₃ is liquefied and added via line132 to liquid NH₃ supplied via line 133, after which the liquid NH₃ isfed to absorber 114. The compounds that are not liquefied in condenser130, such as N₂ and H₂, as well as a small quantity of NH₃, aredischarged as an inert vent stream via line 134.

In FIG. 2, in comparison with the embodiment of FIG. 1, the gaseousmixture containing NH₃, CO₂ and H₂O, which is discharged via line 212from the cooling vessel 204, is fed to condenser 236. In condenser 236partial condensation takes place so that a liquid phase and a gas phaseare formed. The liquid phase and the gas phase are fed via line 238 toabsorber 214.

In FIG. 3, in comparison with FIG. 2, a separator 340 is insertedbetween the condenser 336 and absorber 314. The liquid phase and the gasphase formed through partial condensation in condenser 336 are fed vialine 338 to separator 340. The liquid phase is discharged via line 342and combined with the ammonium carbamate solution discharged fromabsorber 314 via line 322. The combined flow is then discharged via line344, after which the flow is split into a part that is fed to thecooling vessel and a part that is removed. The gas phase coming from theseparator is fed to absorber 314 via line 346.

In FIG. 4, in comparison with FIG. 1, the part of the discharged flow ofgaseous NH₃ to be liquefied is fed via line 428 to compressor 448. Theflow of compressed gaseous NH₃ is subsequently fed via line 450 tocondenser 430, where it is liquefied. The increased pressure makes iteasier to liquefy the NH₃, since the condensation temperature of NH₃increases when the pressure is increased.

FIG. 5 shows the state of the art without the presence of any means ofliquefying a part of the flow of gaseous NH₃ and returning it to theabsorber, and without any means of returning a part of the ammoniumcarbamate solution coming from the absorber to the cooling vessel. Inaddition a part of the flow of gaseous NH₃ discharged from absorber 514via line 520 is discharged via line 552 to a scrubber unit in order toseparate the inert compounds.

The process according to the invention is elucidated further on thebasis of an example and a comparative experiment. The example has beenconducted according to the embodiment described in FIG. 2. Thecomparative experiment has been conducted according to the embodiment ofFIG. 5. See Table 1 and Table 2 for the results. In the tables ammoniumcarbamate is not represented as such but as converted quantities of CO₂and NH₃.

The results indicate that, with melamine-containing gaseous feed streamsof equal quantity and composition (via 202 and 502) and with virtuallyequal quantities of removed CO₂ (via 224 and 524) and removed gaseousNH₃ (via 226 and 526), the quantity of NH₃ that is discharged per unittime via the ammonium carbamate solution (via 224 and 524) issignificantly smaller in the example than in the comparative experiment(7 tonnes/hour and 9 tonnes/hour, respectively). As a result, steamconsumption in downstream steps for liberating NH₃ from the removedammonium carbamate solution drops from 15 tonnes/hour steam for theammonium carbamate solution removed via 524, to 12 tonnes/hour for theammonium carbamate solution removed via 224.

In addition the quantity of H₂O that is removed (via 224 and 524respectively) is significantly lower in the example than in thecomparative experiment (8.6 tonnes/hour and 13 tonnes/hour,respectively); this allows energy savings in utilizing the dischargedCO₂ and NH₃ (for example for the synthesis of urea).

Also the required quantity of liquid NH₃ that needs to be supplied inthe example via 233 is significantly lower than the quantity of liquidNH₃ that needs to be supplied in the comparative experiment via 516 (3tonnes/hour and 9 tonnes/hour, respectively).

Furthermore, the inert vent stream according to the invention which isdischarged via 234 and which in this example contains between 0.001 and0.1 tonne/hour inert compounds such as N₂ and H₂, contains only littleNH₃ (0.02 tonne/hour) so that separating the inert compounds isaccomplished much more easily in comparison with the treatment which isnecessary in the comparative experiment for the flow that is dischargedvia line 552, which flow in relation to the quantity of inert compounds,which also in this comparative experiment is between 0.001 and 0.1tonne/hour, contains very much NH₃ (5 tonnes/hour).

The results also indicate that the N/C ratio in the ammonium carbamatestream discharged from the absorption section according to the inventionvia 222 is 10/8, i.e. 1.25, which is significantly lower than the N/Cratio of the ammonium carbamate stream discharged from the absorptionsection in the comparative experiment via 522, which is 9/6, i.e. 1.5.TABLE 1 Results of the comparative experiment (see FIG. 5) Flow 502 506510 512 516 518 520 522 524 526 538 552 Melamine 5 0 5 0 0 0 0 0 0 0 0 0NH₃ 28 0.9 0.9 28 9 0 28 9 9 23 28 5 CO₂ 6 0.3 0.3 6 0 0 0 6 6 0 6 0 H₂O0 19 6 13 0 0.6 0.002 13.6 13.6 0.002 13 0 Total 39 20.2 12.2 47 9 0.628 28.6 28.6 23 47 5 [tonnes/h] T [° C.] 400 80 117 117 15 15 6 80 80 680 6 P [MPa] 0.6 0.6 0.6 1.6 1.6 0.5 0.5 0.5 0.5 0.5 0.5 Wt % gas 100 00 100 0 0 100 0 0 100 49 100

TABLE 2 Results of the example (see FIG. 2) Flow 202 206 208 210 212 216218 220 222 224 226 228 233 234 238 Melamine 5 0 0 5 0 0 0 0 0 0 0 0 0 00 NH₃ 28 1 3 2 30 8 0 28 10 7 23 5 3 0.02 30 CO₂ 6 1 2 1 8 0 0 0 8 6 0 00 0 8 H₂O 0 14 3 6 11 0 0.6 0.003 11.6 8.6 0.003 0 0.6 0 11 Total 39 168 14 49 8 0.6 28 29.6 21.6 23 5 3.6 0.02 49 [tonnes/h] T [° C.] 400 8074 112 112 29 29 7 74 74 7 7 21 77 P [MPa] 0.6 0.5 0.6 0.6 1.6 1.6 0.50.5 0.5 0.5 0.5 1.6 0.5 Wt % gas 100 0 0 0 100 0 0 100 0 0 100 100 0 10048Note:If NH₃, CO₂ and H₂O are all present in a flow that consists of less than100 wt % gas, said compounds are at least partly present in the form ofan ammonium carbamate solution.

1. Process for preparing melamine, comprising a first cooling step fromwhich a mixture containing NH₃, CO₂ and H₂O is released, wherein: themixture and a flow of liquid NH₃ are fed to an absorption section; themixture and the flow of liquid NH₃ in the absorption section arecontacted with each other, with gaseous NH₃ and an ammonium carbamatesolution being formed; the gaseous NH₃ and the ammonium carbamatesolution are separately discharged from the absorption section, a partof the discharged ammonium carbamate solution is returned to the firstcooling step, wherein a part of the discharged ammonium carbamatesolution is removed, wherein the removed ammonium carbamate solutioncomprises removed CO₂, and a part of the discharged gaseous NH₃ isliquefied in a second cooling step and returned to the absorptionsection, wherein the quantity by weight of discharged gaseous NH₃ whichis liquefied divided by the quantity by weight of removed CO₂ is between0.01 and
 5. 2. Process according to claim 1, wherein the mixture, priorto being fed to the absorption section, is subjected to a third coolingstep, wherein partial condensation takes place whereby a liquid phaseand a gas phase are formed.
 3. Process according to claim 2, wherein theliquid phase is separated from the mixture.
 4. Process according toclaim 1, wherein a compression step is carried out on at least the partof the discharged gaseous NH₃ that is to be liquefied.
 5. Process forpreparing melamine, comprising a first cooling step from which a mixturecontaining NH₃, CO₂ and H₂O is released, wherein: the mixture and a flowof liquid NH₃ are fed to an absorption section; the mixture and the flowof liquid NH₃ in the absorption section are contacted with each other,with gaseous NH₃ and an ammonium carbamate solution being formed havingan N/C ratio; the gaseous NH₃ and the ammonium carbamate solution areseparately discharged from the absorption section, wherein the N/C ratioin the ammonium carbamate solution is reduced to 1.4 or less byreturning a part of the discharged ammonium carbamate solution to thefirst cooling step and by subjecting the gaseous NH₃ and/or the mixtureto a second, respectively third, cooling step.
 6. Process according toclaim 5, wherein the N/C ratio is reduced to 1.3 or less.
 7. Processaccording to claim 5, wherein the N/C ratio is reduced to 1.2 or less.8. Process according to claim 5, wherein the third cooling stepcomprises a partial condensation step on the mixture, prior to themixture being fed to the absorption section, with a liquid phase and agas phase being formed.
 9. Process according to claim 8, wherein theliquid phase is separated from the mixture.
 10. Process according toclaim 5, wherein a part of the discharged gaseous NH₃ is liquefied inthe second cooling step and returned to the absorption section. 11.Process according to claim 10, wherein a compression step is carried outon at least the part of the discharged gaseous NH₃ that is to beliquefied in the second cooling step.
 12. Process according to claim 8,wherein a part of the discharged gaseous NH₃ is liquefied in the secondcooling step and returned to the absorption section.
 13. Processaccording to claim 12, wherein a compression step is carried out on atleast the part of the discharged gaseous NH₃ that is to be liquefied inthe second cooling step.